1. Field of Disclosure
The present disclosure relates generally to wireless communication, and more particularly, to an apparatus and method to reduce delay of Circuit Switched FallBack (CSFB) (voice) Call while operating with Multi/Dual SIMs.
2. Description of the Related Art
Long-Term Evolution (LTE) is a technology proposed by the 3rd Generation Partnership Project (3GPP) which supports high data rate (e.g., 50 Mbps UpLink (UL) and 100 Mbps DownLink (DL)) and is a packet switched system. In a 3GPP based 3G/LTE system, during a combined registration procedure on LTE, a Mobility Management Entity (MME) coordinates with a Mobile Switching Center (MSC) configured to that MME and provides the Location Area Identified (LAI) and Temporary Mobile Subscriber Identity (TMSI) to connect a Circuit Switched (CS) domain directly in a GSM (Global System for Mobile communication) Edge Radio Access Network/Universal Terrestrial Radio Access Network (GERAN/UTRAN) without registration. In the case where a User Equipment (UE) is in LTE and a Mobile Originated/Mobile Terminated (MO/MT) Call is triggered, the UE initiates an Extended Service Request Procedure. As a response to this procedure, the network will trigger a Redirection Request based on the MSC to which it is connected and the LAI information it has provided in an Attach/Tracking Area Update (TAU) Accept message.
However, if the UE is in LTE and an MO/MT Call is initiated, the Extended Service Request Procedure can fail because of one or more of, but not limited to, a lower layer failure, network congestion, a connection failure, and the like. According to current specifications, and the nature of a packet switched system, only burst data is processed by the UE. In order to support a voice call in a multimode device capable of LTE/3G/2G, 3GPP has developed the CS Fallback mechanism to fall back to 2G or 3G for a voice call, wherein a CS Fallback supported UE should switch to GERAN/UTRAN and proceed with the Mobility management (MM) and General Packet Radio Service (GPRS) MM (GMM) specific procedures.
When a user attempts to make a normal/emergency call, and when the UE is on an Evolved UTRAN (EUTRAN) network, a CSFB Call Request may fail because of various reasons, such as, but not limited to, temporary causes like MSC Not Reachable, network congestion, temporary radio related problems (like random access failure, Radio Resource Control (RRC) connection failure) or permanent causes (CS domain not available), and any other causes as mentioned in the 3GPP specification. The main problem is selecting the right Radio Access Technology (RAT). In EUTRA, an Attach Accept network provides only LAI information but does not provide any RAT information where the provided LAI is valid. In this case, the UE may select a High Priority RAT based on Subscriber Identity Module (SIM) Home Public Land Mobile Network (HPLMN) files and proceeds with the same RAT. If it is not present in the SIM, the UE may select the RAT randomly and proceed with a Public Land Mobile Network (PLMN) scan. In this case, if the UE selects an incorrect RAT, the call might be delayed significantly.
FIG. 1 is a flow diagram 100 illustrating a procedure between a network and a UE during an Extended Service Request (ESR) failure for a CS call, according to the related art.
Referring to FIG. 1, the network transmits an Evolved Packet System (EPS) Attach Accept message to the UE along with LAI information, wherein the LAI information comprises location and area details of the network. Upon receiving the EPS Attach Accept message, the UE can initiate a Mobile Originated (MO) voice call. While initiating the MO voice call, the UE can transmit an Extended Service request (ESR) to the network for making the voice call.
The network can experience a lower layer failure and thus can send a RRC Connection Release message to the UE, and thereby the UE stops the voice call. Upon receiving the RRC Connection Release message, the UE determines that the RAT providing the GERAN/UTAN has a lower layer failure problem and must switch to another RAT for network access. As the UE does not have any other secondary RAT to rely on, the UE randomly searches for a GERAN/UTRAN RAT to camp on. The UE can initiate a PLMN scan on a selected RAT to identify a suitable RAT to camp on. Scanning and searching for a suitable RAT can be a time consuming process and until a RAT is identified, the UE will be without any RAT to camp on. Once the UE identifies a suitable RAT, the UE sends a Cubic Metric (CM) service request to the selected RAT. According to the present procedure, the UE does not have any specific information regarding which RAT is to be selected, and the selection is done randomly.
FIG. 2 is a schematic flow diagram 200 illustrating a procedure between a network and a UE, wherein the lack of RAT information causes delay in RAT selection, according to the related art.
Referring to FIG. 2, the network transmits an EPS Attach Accept message to the UE along with LAI information, wherein the LAI information comprises of location and area details of the network. Upon receiving the EPS Attach Accept message, the UE can initiate a MO voice call. While initiating the MO voice call, the UE can transmit an ESR to the network for making the voice call.
The network can experience a lower layer failure and thus sends a RRC Connection Release message to the UE, and thereby the UE stops the voice call. Upon receiving the RRC Connection release message, the UE can determine that the RAT providing the GERAN/UTAN has a lower layer failure problem and must switch to another RAT for network access. As the UE does not have any other secondary RAT to rely on, the UE randomly searches for a GERAN/UTRAN RAT to camp on. The UE can initiate a PLMN scan on a UTRAN to identify a suitable RAT to camp on. The network responds to the UE with a message indicating that the UTRAN is not available for switching, and the UE can switch to any of the 2G networks for accessing the network. The UE can initiate scanning on a GERAN upon receiving the message from the network. The process of scanning for the UTRAN, receiving the message from the network, and initiating the scanning of a GERAN can cause delay in identifying a suitable RAT. Once the UE identifies a suitable RAT, the UE sends a CM service request to the selected RAT. According to the present procedure, after ESR procedures fails due to a lower layer failure, the UE selects the 3G RAT, but the network can be configured with only LTE and 2G networks. So a long delay can be experienced before the call is established.
So, in case of temporary or radio failure causes according to the 3GPP specification (e.g. 3GPP 24.301 TS), it is mentioned that the UE should move to a GERAN/UTRAN. Currently, there is no method to identify to which RAT a UE should move in case of a voice call pending, while the UE was on LTE. Therefore, maintaining the mapped LAI, SIM and RAT information when the UE successfully performs registration using one SIM will help the other SIMs to camp on to a cell immediately and process the CS call without a long delay.
Thus, there is a need for a system and method that can obtain RAT related information on a UE side locally using the CSFB call success information when a call is triggered on one SIM to overcome the problems discussed herein above.